The cellular basis of the impaired immune responses of human lymphocytes from aged donors will be determined. The capacity of human T lymphocytes to proliferate in culture with plant lectins and the capacity of B lymphocytes to form hemolytic plaques following culture with pokeweed mitogen will be measured. Similar studies will be carried out in aged mice. These studies in experimental animals will permit the role of thymic hormone decline in the age-associated immunodeficiencies to be determined. The immune function of lymphocytes from old animals exposed in vivo or in vitro to thymopoietin will be tested.